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Try this
Powe up with the monitor

Hit the Windows key and the R keys at the same time.
Type msconfig
Hit enter, click on boot, select Safe mode, then save and exit, disconnect the monitor...

So you can boot normally to a monitor but it won boot into safe mode?

Same issue in safe mode?

Have you tried thD button ‘test’?
The cable could have become unseated - does the spash logo come from a different source to the display - a graphics card

My thoughts too
Any mileage in wiping it and selling it - buying something new?
Can you get into the bios with an attached monitor ?

Have you pressed down the D button while it powers on?

The screen obviously works as it’s showing the dell spash screen
Sure this is not a bios/HDD issue
Try this
https://www.youtube.com/watch?v=njAvrRG4hWM